Solved.
Remember
this thread?
I had him try to trade me a rusty rapier, sure enough he went LD immediately.
So I killed a skeleton and looted some bone chips, handed them to him. He's now fixed.
So there is a bug occuring that's specific to individual characters, and independent of the files on their computer, that will cause them to DC every time they initiate a trade, until someone else initiates a trade with them, at which point the bug is permanently fixed.